The invention relates to a method for gluing plywood and LVL (long veneer lumber) by assembling several layers of veneers one over the other and curing the adhesive placed between the single layers by pressing in a heated press.
LVL and plywood boards consist of veneers with thicknesses of 0.5 mm to 6 mm. The veneers are glued, as a rule, with phenol formaldehyde adhesive, assembled together, compressed in a hot press and the glue is cured by the action of heat. Phenol formaldehyde adhesive is a high-temperature glue which, after reaching 110xc2x0 C. hardens in about 1 minute by condensation reactions. At temperatures lower than 110xc2x0 C. the condensation reaction takes substantially longer and takes place to only a limited extent. The strength of this glue bond is therefore less than when it is cured above 110xc2x0 C. If the curing reaction is properly controlled, the phenolic resin glue produces a waterproof and boil-proof bond. The plywood and LVL produced are weatherproof and can be used in construction work. The adhesive is good in comparison to other adhesives with which a weatherproof and boil-proof bond can be achieved. Therefore, for plywood and LVL used in construction work, phenol formaldehyde glue is used almost exclusively.
Plywood and LVL boards are produced as a rule in thicknesses up to 45 mm. The veneers and glue are heated exclusively by conduction in the hot press. The heat passes from the press platens through the outer layers into the inner layers. The heating time, i.e., the time elapsing until 110xc2x0 C. is reached in the center of the board, increases disproportionately with increasing board thickness due to the poor thermal conductivity of wood. For example, at an initial board temperature of 30xc2x0 C. and a press platen temperature of 160xc2x0 C., the heating time for 20 mm thick boards is 5 minutes, for 40 mm thick boards 20 minutes, and for 80 mm thick boards 120 minutes.
In order also to produce boards of thicknesses greater than 45 mm (thick boards) in an economically reasonable time various proposals have been made. Increasing the press platen temperature to above 160xc2x0 C. has proven disadvantageous because at temperatures higher than 160xc2x0 C. for periods of over 20 minutes the wood is thermally decomposed. Also the thermal decomposition definitely reduces the strength of the board. In a known discontinuous manufacturing process the boards are heated in single-stage or multi-stage presses by high frequency. The press platens are designed so that they can be heated and used simultaneously as condenser platens. The press platen temperature in this process is about 110xc2x0 C. The veneers are deposited onto the bottom press platen, compressed, and then heated by high frequency over the entire pad cross section. To operate such an apparatus complicated protective measures are necessary. The press must be shielded. Only certain frequencies can be emitted. Moreover, trained personnel are needed during operation. Since the efficiency of the electric energy is around 50%, this way of heating involves high energy costs. Furthermore, in the case of excessively great veneer moisture content there is the danger of burning through and carbonizing the board. In a continuous manufacturing process heating with high frequency cannot be done since the continuous hot presses cannot be equipped for high-frequency heating.
According to DE 197 18 772, in the continuous manufacturing process the veneers are heated ahead of the hot press by microwave irradiation across the entire cross section of the mat. The production of very thick boards by this method involves some disadvantages. Since the microwave is a few meters ahead of the hot press, premature setting of the adhesive can occur while the veneers are transported on the way from the microwave to the hot press. The adhesive bond is weakened by premature setting. Since the production speed decreases with increasing thickness, such premature setting also increases. In the event of a production halt, this kind of great premature setting of the veneers situated between the microwave and the hot press can occur such that the section of boards is wasted.
Up to a certain temperature and moisture combination the wood is softened as the temperature and moisture increase. With a common veneer moisture content of about 8-10%, the wood definitely softens at temperatures above 100xc2x0 C., and the softening can increase further up to about 140xc2x0 C. While the glue is setting the veneers must be subjected to a certain minimum pressure of about 1.5 N/mm2 to achieve full-surface contact. The thickness of the veneers is reduced by the pressure while it is applied (press shrinkage). As the temperature of the veneers increases the press shrinkage increases disproportionately after a temperature above 100xc2x0 C. The press shrinkage in the normal hot pressing of LVL and plywood boards is about 10%. Press shrinkage is undesirable, as a rule, since more material must be put in for the same order of magnitude. Another problem is that, in the finished board, the compressed veneers spring back due to swelling and shrinking processes. This rebound makes the board irregular in thickness and in extreme cases it must be discarded.
As a disadvantage of the conventional methods it is also to be stated that, during the hot pressing the temperature of the veneers rises to above 100xc2x0 C., water is evaporated, and an excess steam pressure builds up between the veneers. As the moisture of the veneers increases, the excess steam pressure rises. If the excess steam pressure between the veneers is greater after leaving the press than the strength of the glue bond, the board bursts and becomes waste. This bursting occurs as a rule in the center of the board, since this layer is not heated until the end of the pressing operation and thus receives the briefest curing time and has the lowest strength of adhesion. Also, in the outer layers the steam can flow partially over the surface of the board after it leaves the hot press, so that the steam pressure is reduced in that area.
The invention is addressed to the problem of producing thick LVL or plywood boards economically such that additional heating besides the conductive heating in the hot press by high frequency, for example, or preheating, is no longer necessary.
The solution of the above described problems consists in the fact that the gluing and pressing of the outer layers of a veneer pack to form LVL or plywood boards is performed by means of high-temperature adhesive and the inner layers by low-temperature adhesive, so that the amount of heat to be put into the veneer pack diminishes from the outside in.
Additional advantageous measures and embodiments of the subject matter of the invention will be found in the subordinate claims and the following description with the drawing.